There has been known an organic electroluminescence device (hereinafter, occasionally referred to as an “organic EL device”) that includes an organic compound layer (in which an emitting layer is included) between an anode and a cathode and emits light using exciton energy generated in the organic compound layer by a recombination of holes injected into the emitting layer from the anode and electrons injected into the emitting layer from the cathode.
Such an organic EL device, which has the advantages as a self-emitting device, is expected to serve as an emitting device excellent in luminous efficiency, image quality, power consumption and thin design.
In an optical design of the organic EL device, adjustment of an optical interference distance and the like are conducted in order to improve a luminous efficiency. By adjusting a film thickness of the organic compound layer (e.g., hole transporting layer), an effective luminous efficiency can be improved and emission spectrum can be modulated. Thus, the adjustment of the optical interference distance is a requisite means for the device design.
However, the adjustment of the optical interference distance is not sufficient for extracting light trapped within the device. Accordingly, an arrangement of the device for efficiently extracting the light trapped within the device to improve the luminous efficiency has been studied.
In a device arrangement in which light is extracted toward a light-transmissive support substrate (light-transmissive body) supporting the organic compound layer, the light is mainly lost in the following modes.
(i) Substrate Mode
A substrate mode is a mode of the light trapped within the light-transmissive body due to total reflection of the light at an interface between the light-transmissive body and air.
(ii) Thin Film Mode
A thin film mode is a mode of the light trapped in a transparent electrode and the light-transmissive body due to total reflection of the light at an interface between the transparent electrode and the light-transmissive body.
(iii) Surface Plasmon Mode
A surface plasmon mode is a mode of the light of which luminous energy is absorbed in a metal electrode as surface plasmon. The term “surface plasmon” refers to waves that are localized at an interface between metal and a dielectric in a form of a combination of charge compression waves propagated along the interface and electromagnetic waves accompanying the charge compression waves. “Plasmon” is an abbreviation for “Plasmon Polariton” representing a phenomenon that some force is applied on free electrons evenly present in metal to generate and propagate polarization waves. In such a phenomenon, a particular instance where the polarization waves are propagated along an interface between metal and a dielectric is referred to as the surface plasmon or surface plasmon polariton.
Light loss in the above modes accounts for from several tens to almost 100 percent of a total luminous energy in an organic emitting layer depending on existing conditions of luminous molecules. Accordingly, extraction of the light in such loss modes is a requisite for development of an organic electroluminescence device exhibiting a highly efficient emission
Light is trapped in the substrate mode, thin film mode and surface plasmon mode because the light radiated from the emitting layer is trapped within the organic electroluminescence device as evanescent light. It is considered to be the most difficult to extract the light in the surface plasmon mode as compared with the other optical modes. Some specific methods of extracting the light in the surface plasmon mode have been reported.
For example, Patent Literature 1 discloses that the light in the surface plasmon mode can be a useful radiation in a form of Bragg scattering with use of Bragg diffraction.